1. Field of the Invention
The present invention relates to a transconductor circuit, more particularly to a CMOS transconductor circuit.
2. Description of the Prior Art
Transconductor circuits are the most basic components in transconductor-capacitor filters, or Gm-C filters, with their performances directly affecting those of transconductor-capacitor filters or Gm-C filters. Therefore it is most important to promote the performance of a transconductor circuit, in designing a transconductor-capacitor filter. As normally used in an open loop circuit, a transconductor can""t take advantage of negative feedback as general negative feedback closed loop circuits to increase the linearity of the circuit.
As a result of a circuit composed of a transconductor, for example, a Gm-C filter often has a low linearity compared with a negative feedback circuit composed of an amplifier, for example, an active RC filter. Nevertheless, featuring higher frequency operation than that of a closed loop circuit, an open loop circuit such as a transconductor-capacitor filter composed of a transconductor is often used in high frequency (100 MHz range) operations with a low linearity requirement ( less than 50 dBc).
But in low frequency and high linearity operation (1 MHz range,  greater than 65 dBc), it is an active RC filter that is preferred. In 10 MHz range and moderate linearity (60 dBc), however, the two structures have their advantages and disadvantages, respectively. As a transconductor-capacitor filter composed of a transconductor in which the linearity of the transconductor determines the linearity of the transconductor-capacitor filter, increasing the linearity of the transconductor becomes first and foremost. Hence it is desirable to present a transconductor circuit with the features of high frequency operation and high linearity.
FIG. 1 shows a transconductor circuit that employs a local feedback loop (refer to: Z. Y. Chang, D. Haspeslagh, and J. Verfaillie, xe2x80x9cA highly linear CMOS Gm-C bandpass filter with on-chip frequency tuning, xe2x80x9cIEEE JSSC, Vol. 32, No. 3, pp.388-397, March 1997.). As shown in FIG. 1, that a negative feedback loop comprises a differential amplifier 100 and input PMOS transistor 102 so as to create negative feedback connects VXP to VIP. In the same way, VXN is connected to VIN. As a result, a current xcex94I flows through the resistor with xcex94I=xcex94V/R=(VIPxe2x88x92VIN)/R. Accordingly, a current Ixe2x88x92xcex94I flows through PMOS transistor 102 resulting in a current xe2x88x92xcex94I=xe2x88x92(VIPxe2x88x92VIN)/R=Gm*(VIPxe2x88x92VIN) on the output, with the transconductance Gm of the transconductor equal to 1/R. It can be seen that the transconductance of the transconductor equals the linear resistance by way of negative feedback loop and is highly linear.
Notwithstanding, the linearity of the Gm-C integrator, as shown in FIG. 1, the transconductor circuit is limited for the sake of the integrated capacitor that is not in the negative feedback loop of the transconductor circuit. Furthermore, the local negative feedback gain of the transconductor circuit is limited to the differential amplifier 100 while the input range of the transconductor circuit is limited to the common mode input range of the differential amplifier 100 with limited gain. Therefore it counts to achieve a higher gain of the differential amplifier 100 to improve the linearity of the transconductor circuit.
According to the various shortcomings such as low linearity of the traditional transconductor circuits exhibited in the aforesaid background of the invention, it is an object of the present invention to provide a CMOS transconductor circuit with the features of high frequency operation and high linearity.
Further, the CMOS transconductor circuit with high linearity provided in the present invention works steadily in operations of high frequency and high linearity thus, achieving the compatibility of high frequency and high linearity operation, that the well-known transconductor circuits can not offer.
Owing to low linearity and high adaptability in high frequency operation of general transconductor circuits, compared with negative feedback loop circuits composed of amplifiers, transconductor-capacitor filters composed of general transconductors are normally used in high frequency operations without high linearity requirement.
Accordingly, the CMOS transconductor circuit with high linearity provided in the present invention is to utilize a local negative feedback loop in place of a general transconductor circuit using an open loop to achieve the features of high frequency operation and high linearity.
According to the aforesaid object, the present invention provides a CMOS transconductor circuit, which comprises an output transistor, with the gate electrode and the source electrode of the output transistor connected with a second and a first current source as an input respectively and the drain electrode of the output transistor as the output of the current; a resistor, which couples to the source electrode of the output transistor; and a local negative feedback loop which connects the gate electrode of the output transistor to the source electrode of the output transistor to set the transconductance of the CMOS transconductor circuit the equal of the reciprocal of the resistance of the resistor.
Nevertheless the aforesaid local negative feedback loop comprises a first transistor with the source electrode of the first transistor connected to the source electrode of the output transistor; a second transistor with the drain electrode of the second transistor connected to the gate electrode of the output transistor; and a third transistor with the source electrode of the third transistor connected to the ground and the drain electrode of the third transistor connected to the source electrode of the second transistor while the drain electrode of the first transistor connected to the source electrode of the second transistor and the drain electrode of the third transistor at the same time. The present invention further comprises a fourth transistor so as to connect the drain electrode of the first transistor to the drain electrode of the fourth transistor and the gate electrode of the fourth transistor to the gate electrode of the third transistor.
As described above, the CMOS transconductor circuit provided in the present invention utilizes a local negative feedback loop in place of a general transconductor circuit feedback loop to achieve features in high frequency operation requiring high linearity. Further, with features of high frequency operation requiring high linearity, the CMOS transconductor circuit provided in the present invention can be applied to design a CMOS transconductor-capacitor filter with the features of high frequency operation and high linearity.